In-situ DRIFTS measurements for the mechanistic study of NO oxidation over a commercial Cu-CHA catalyst

We report a mechanistic DRIFTS in-situ study of NO₂, NO+O₂ and NO adsorption on a commercial Cu-CHA catalyst for NH₃-SCR of NO_x. Both pre-reduced and pre-oxidized catalyst samples were investigated with the aim of clarifying mechanistic aspects of the NO oxidation to NO₂ as a preliminary step towards the study of the Standard SCR reaction mechanism at low temperatures. Nitrosonium cations (NO⁺, N formal oxidation state=+3) were identified as key surface intermediates in the process of NO (+2) oxidation to NO₂ (+4) and nitrates (+5). While NO⁺ and nitrates were formed simultaneously upon catalyst exposure to NO₂, nitrates evolved consecutively to NO⁺ when the catalyst was exposed to NO+O₂, suggesting that nitrite-like species, and not NO₂, are formed as the primary products of the NO oxidative activation over Cu-CHA. Upon catalyst exposure to NO only, i.e. in the absence of gaseous O₂, NO⁺ and then nitrates were formed on a pre-oxidized sample but not on a pre-reduced one, which demonstrates the red-ox nature of the NO oxidation mechanism. The negative effect of H₂O on NO⁺ and nitrates formation was also clearly established. Assuming Cu dimers as the active sites for NO oxidation to NO₂, we propose a mechanism which reconciles all the experimental observations. In particular, we show that such a mechanism also explains the observed kinetic effects of H₂O, O₂ and NO₂ on the NO oxidation activity of the investigated Cu zeolite catalyst.